Radio frequency amplifiers generally comprise a transistor, an input circuit, an output circuit, and components which set the bias point of the transistor. The transistor is a three terminal device that includes a signal terminal, an output terminal, and a return terminal. In a typical configuration, the transistor accepts radio frequency signals into its signal terminal, amplifies the signals based on the output terminal to return terminal impedance ratio, and transmits the amplified signal from its output terminal. The input and output circuits are used to provide amplifier selectivity, impedance matching to drive and load circuitry, and amplifier gain control. Typically, the input circuit may be utilized to match the transistor's input impedance to a driving circuit, such as an antenna or a driving transistor's output impedance, and the output circuit may be used to match the transistor's output impedance to a load circuit, such as the input to a mixer or a transistor's input impedance.
As is known, radio frequency amplifiers are inherently nonlinear circuits due to the transistor's internal composition. A radio signal entering the transistor's signal terminal may produce undesired output frequency components at harmonics of the radio signal if the radio signal strength is strong enough to drive the transistor out of its linear region. Common problems, such as harmonic distortion and intermodulation distortion, are also encountered due to nonlinear effects in radio frequency amplifiers. Harmonic distortion occurs when a strong radio signal at a frequency of one half the desired frequency enters the signal terminal of the transistor to produce an unwanted second harmonic output signal at the desired frequency. Intermodulation distortion occurs when two radio signals, one desired and one undesired, enter the signal terminal of the transistor and produce an unwanted output signal within the selectivity of the amplifier.
Common amplifier design practice relies on a parallel resonant circuit located between the transistor's output terminal and a DC voltage source to provide the amplifier's gain and frequency selectivity. Typically, the parallel resonant circuit includes an inductor in parallel with a voltage variable capacitor, or varactor, to allow DC voltage control of the amplifier's center frequency. The varactor comprises a reverse biased diode whose capacitance varies as a function of the DC bias.
Although simple in topology and implementation, the parallel resonant circuit has several shortcomings. First, the loaded quality factor, Q, of the parallel resonant circuit decreases as the amplifier's center frequency increases. This result degrades the selective bandwidth of the amplifier when the varactor capacitance decreases. Secondly, the amplifier provides limited rejection of harmonic distortion; typically in the 60 dB range. An additional 10 to 20 dB of rejection may be necessary for desired receiver sensitivity. Finally, AC voltage excursions exhibited across the varactor, due to the output radio signal, cause instantaneous capacitance variations. These variations produce additional output signal distortion which is even greater when a small DC voltage is applied across the varactor.
To overcome some of the parallel resonant circuit's performance issues, the amplifier may include a series resonant circuit incorporated between the transistor's return terminal and a signal common. Typically, the series resonant circuit includes an inductor in series with a varactor to permit DC control of the amplifier's center frequency. Contrary to its parallel counterpart, the loaded Q of the series resonant circuit increases as the amplifier's center frequency increases; thus, the bandwidth of the amplifier remains constant as varactor capacitance decreases. Additionally, the series resonant circuit improves rejection of harmonic distortion by reducing the amplifier's gain at the half carrier frequency. This also prevents the amplifier from leaving its linear operating region and creating harmonic output signals.
Although the series resonant circuit solves two of the three critical performance issues encountered when utilizing the parallel resonant circuit approach, it also creates new problems. First, in the typical configuration, the amplifier necessitates a DC current path from the transistor's return terminal to the signal common in order to allow a return path for the DC current. The presence of the varactor in the series resonant circuit prevents DC current flow from the transistor's return terminal to the signal common. Further, in order to attain the desired amplifier selectivity, the reactive component impedances utilized in the series resonant circuit may be considerably larger than those used in the parallel resonant circuit approach. Depending on amplifier center frequency, obtaining usable series resonant circuit components may become an obstacle due to their extraordinarily large or small values. Finally, to provide amplifier gain comparable to that obtained with a tuned parallel resonant circuit, the impedance of the transistor's return terminal must remain small enough such that the output terminal to return terminal impedance ratio for the amplifier with the tuned series resonant circuit is similar to that for the amplifier with the tuned parallel resonant circuit.
Therefore, a need exists for a radio frequency amplifier that provides the advantages of both the series resonant circuit and the parallel resonant circuit without their respective limitations.